In off-grid photovoltaic systems, the power of the off-grid inverter is generally determined based on the user's load type and power.
Loads are classified according to their impedance characteristics into three categories: resistive loads, inductive loads, and capacitive loads. Resistive loads: Loads where the current and voltage have no phase difference are resistive loads, such as rice cookers, light bulbs, electric stoves, and soldering irons. Inductive loads: Loads where the current lags the voltage by one phase difference are inductive loads, such as washing machines, air conditioners, refrigerators, water pumps, range hoods, and other loads with motors, as well as transformers, relays, and compressors. Capacitive loads: Loads where the current leads the voltage by one negative phase difference are capacitive loads, such as compensation capacitors, computers, televisions, and other switching power supplies used in homes. The starting power of inductive loads such as motors is 5-7 times their rated power; this starting power must be taken into account when calculating the inverter's power.
If the load is something like an elevator, it cannot be directly connected to the inverter output. This is because when the elevator descends, the motor reverses, generating a back electromotive force that can damage the inverter if it enters the system. If an off-grid system is necessary, it is recommended to add a frequency converter between the inverter and the elevator motor.
If the photovoltaic system is only used for pumping water and a water tower can be built, it is recommended to use a photovoltaic pumping inverter, which can save costs. For example, a 2P water pump would require a 5kW or higher off-grid inverter to drive it if a regular inverter is used, and it would also need a battery of 200AH or higher, with a total cost of more than 10,000 yuan. However, if a pumping inverter is used, only a 2kW inverter is needed, and the price is less than 3,000 yuan.
Inverter waveforms are mainly divided into two categories: sine wave inverters and modified sine wave inverters. Modified sine wave inverters use PWM (Pulse Width Modulation) to generate the modified wave output. Due to approximately 20% harmonic distortion, they cannot power inductive loads such as air conditioners, but can power resistive loads such as light bulbs. Modified sine wave inverters use non-isolated coupling circuits, resulting in simpler components and higher efficiency. Pure sine wave inverters use isolated coupling circuit designs, which are more complex and costly. They can connect to any common electrical appliance (including televisions, LCD monitors, and especially inductive loads such as refrigerators) without interference.
Inverters are classified into high-frequency and low-frequency types based on isolation. A low-frequency inverter has a 50Hz low-frequency isolation transformer after it; a high-frequency inverter uses high-frequency switching technology, replacing the low-frequency transformer with high-frequency switching elements or a high-frequency isolation transformer. Low-frequency inverters offer advantages such as stable and reliable power components, strong overload capacity, and high impact resistance. Their disadvantages include lower efficiency, heavier weight, and higher price. High-frequency inverters come in both high-frequency isolated and non-isolated versions. Their advantages include smaller size, higher efficiency, and lower price. Their disadvantage is slightly lower impact resistance. If the load is an inductive load with high impact (such as a motor) that is not frequently moved, a low-frequency inverter is recommended. If the load is a resistive load with low impact that is frequently moved, a high-frequency inverter is recommended.
The inverter's output power must be greater than the load's power. For demanding applications such as monitoring stations and communication stations, the output power is the sum of all load powers. However, for ordinary impoverished households, considering that all loads cannot be turned on simultaneously, a factor of 0.7-0.9 can be multiplied by the sum of load powers to save costs.
